1. Field of the Invention
This invention relates to the preparation of synthetic fuels. More particularly this invention relates to the preparation of high density, high energy, low viscosity liquid hydrocarbon fuel for use in jet or rocket propulsion of missiles or aircraft. Still more particularly this invention relates to the conversion of endo-isomers within RJ-4 fuel stock to exo-isomers by contacting said fuel stock with a catalyst at room temperature thus resulting in a lower freezing-lower viscosity fuel.
2. Description of the Prior Art
High energy, high density liquid fuels are particularly demanded in aircraft and missile applications where fuel carrying capacity is very limited. They are useful in both jet propulsion systems such as ramjet, turbo-jet and pulse jet, and in rockets, i.e., devices containing their own oxygen or oxidizing agent. Missile applications are particularly stringent since no refueling can be accomplished. Range is thus dependent upon internal fuel supplies.
Requirements of extended range for air launched missiles while retaining overall dimensions of previous missiles to allow launch from existing aircraft stations has led to the proposal to add ramjet propulsion to previously all solid rocket propulsion in the missile system. Also the introduction of long range subsonic cruise missiles has made extended range a critical operational parameter. High energy, high density liquid hydrocarbon fuels have been proposed and employed in such systems.
High energy, high density fuels suitable for use must exhibit properties such as high volumetric heating value, low viscosity, five year storage capability, excellent combustion characteristics, low toxicity, low cost, low freezing point, flash point no lower than 60.degree. C., compatibility with common materials, and absence of ionizable compounds. Heating value, viscosity, and cost are primary factors in choosing a fuel for ramjet application. Many proposed high density, high energy fuels have exhibited unacceptable viscosity at typical operating temperatures, with viscosities tending to increase significantly with decrease in temperature. Air launched systems are particularly susceptible to fuel viscosity increases since a missile may be cold soaked at high altitude for a considerable length of time before launch. Operations in cold weather or arctic regions are also threatened by high viscosity fuels for both air and ground launched missiles. It is difficult for fuel systems to deliver high viscosity fuels to the engine, and ignition of the fuel is difficult and uncertain. High viscosity of fuels could lead to unacceptable missile failure rates.
A fuel presently used in missile systems is RJ-4, a synthetic hydrocarbon fuel consisting of a production run mixture of the various isomers of tetrahydrodimethyldicyclopentadiene (THDiMeDCPD). It is known that certain isomers of THDiMeDCPD known as endo-isomers tend to have a significantly higher freezing point and viscosity than other isomers known as exo-isomers. U.S. Pat. No. 4,107,223 to Schneider et al. and references cited therein, the text of which is incorporated by reference herein, recognize this fact and provide for a process to convert endo-THDiMeDCPD to a lower viscosity exo-THDiMeDCPD by treatment with AlCl.sub.3 catalyst. The specific isomers employed in the Schneider et al. patent as starting materials are very costly and thus the product is impractical for use as an operational fuel.